Fake shirts, flags and other souvenirs are available on South Africa's streets - often sold by traders at traffic lights.
South Africans have been encouraged to buy national team shirts to wear on special "Football Fridays" but many complain bitterly about the high cost of official Adidas jerseys. Copies sold by hawkers are popular.
"Despite efforts to clamp down on counterfeit goods coming mainly from China and other Asian countries, fake jerseys and other merchandise for national teams will be costing bonafide suppliers millions of dollars in lost revenue," the International Authentication Association (IAA) said in a statement.
The association was established to lead the fight against counterfeiting and represents many global brands.
South African customs officials and police have found large stashes of fake goods at Johannesburg airport and elsewhere, including $2.5 million worth of national team shirts.
IAA chairman Jim Rittenburg said many top sports brands were being hit hard by cheap imitations.
"The problem of fake goods at this ... World Cup is a big concern," he said.
"We are urging all those involved in the fight against counterfeiting, from anti piracy and law enforcement agencies to official suppliers to review their security plans."
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